From:http://www.akadia.com/services/ora_optimize_undo.html
Overview
Starting in Oracle9i, rollback segments are re-named undo logs. Traditionally transaction undo information was stored in Rollback Segments until a commit or rollback statement was issued, at which point it was made available for overlaying.
Best of all, automatic undo management allows the DBA to specify how long undo information should be retained after commit, preventing [...]
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